Move Coq realizations from a .ml file to a driver file.

Note that the file is still generated at compilation time.
The "realized" meta takes two arguments. The first one is the path+name of
the theory, the second one is the translation of it for the target prover.
The meta is supposed to be put into a printer file, so there is no
ambiguity on the target. The second argument can be left empty if it can be
inferred from the first one.
Note that the first argument is not really satisfactory, since it is
redundant with the theory part of the driver. Moreover, its handling is a
bit crude: it does not take into account rich qualifiers and it does not
generate proper error messages if it does not match the theory.